In most peoples' minds Metallica came after Iron Maiden, and while technically true (Maiden formed in '79 and Metallica in '81, Maiden's debut came out in '81 and Metallica's in '83), the bands existed and made releases more or less contemporaneously in the 1980s, especially some of their most important albums (Powerslave and Ride the Lightning both came out in 1984). How come it was Metallica though and not Maiden or the NWOBHM bands that precededed them (such as Weapon, linked below) that innovated the metal genre in such a spectacular way? None of the NWOBHM bands had the crunchy thrash riffs that Hetfield was putting out on Kill 'Em All (think intro to No Remorse) and Ride the Lightning.